Backlit illuminated displays are a widely employed means for presenting visual information to a user where low ambient light conditions may be encountered, and are commonly used in such applications as automobile and aircraft instrument panels. These displays generally present visual information to a user in the form of light colored indicia (letters, words, symbols, etc.) on a dark background; and consist of a display panel bearing the indicia and one or more lamps or other light sources disposed behind or recessed into the back surface of the display panel. In high ambient light conditions, the indicia on the display panel are viewed by light reflected in the front surface of the display panel; however, in low ambient light conditions, light from the lamps behind or recessed into the back surface of the display panel projects through the indicia to provide illumination to the display panel, and the indicia are viewed by this transmitted light. In both high and low ambient light conditions, a high contrast between the indicia and their background (typically greater than about 9:1, as measured by MIL-P-7788F) is desirable; and since this contrast is provided by light transmitted through the indicia in low ambient light conditions, an appropriate brightness of the illuminated areas of the display (typically about 2-9 nit) is desirable.
A typical display panel is prepared from a transparent display panel substrate (a sheet of a transparent polymer, such as a cast acrylate) by:
(a) machining the substrate to the desired shape, and making cutouts in the substrate for switches, instruments, fasteners, and the like to be mounted through the panel; PA1 (b) coating the back surface and side surfaces of the substrate first with a diffuse translucent layer, typically by spray painting of an extremely flat white paint of a thickness between about 15 and 50 microns, to reflect light leaking from the back surface of the substrate back into the substrate in a diffuse manner, then with an opaque layer, typically by spray painting of a flat black paint of a similar thickness, to block remaining light leakage from the substrate; PA1 (c) drilling recesses in the back surface of the substrate for lamps to provide the illumination of the display; PA1 (d) coating the front surface of the substrate with a diffuse translucent layer (also typically by spray painting of a flat paint); and PA1 (e) further coating the front surface of the substrate with an opaque layer (also normally by spray painting), and exposing the underlying translucent layer in a pattern defining the indicia desired in the display. This last step is typically accomplished by completely coating the front surface with the opaque layer, then etching or otherwise selectively removing that opaque layer to expose the translucent layer in the desired pattern. PA1 (a) providing a transparent display panel substrate having a front surface, a back surface, and side surfaces, the substrate being coated with a diffuse reflective layer and overcoated with an opaque layer on the back surface and side surfaces, and having recesses in the back surface to receive lamps to provide illumination to the display; PA1 (b) coating the substrate on the front surface with a diffuse translucent layer of defined thickness; PA1 (c) illuminating the substrate by lamps positioned within the recesses in the back surface of the substrate; PA1 (d) forming an image indicative of the brightness at each point of the front surface of the illuminated substrate and determining from the image the additional thickness of diffuse translucent layer to be added to each point on the front surface of the substrate to produce a uniform brightness of a desired intensity at each point of the front surface when that layer is added and the substrate is illuminated; and PA1 (e) adding to each point on the front surface of the substrate a diffuse translucent layer of a thickness determined for that point in accordance with step (d) above.
U.S. Pat. No. 5,456,955 (Muggli) describes such a process, where the translucent layer on the front surface is overcoated with a clear layer before being further coated with the opaque layer, and the opaque layer is etched with a neodymium yttrium aluminum garnet (NdYAG) laser. The use of a clear layer is stated to simplify the use of the NdYAG laser to etch the opaque layer without potential damage to the underlying translucent layer. The disclosure of this patent is incorporated by reference.
When the display is illuminated so that the indicia are viewed by transmitted light, it is desirable that the intensity of illumination of the various indicia be uniform throughout the display, to minimize the visual distraction caused by "hot spots" of excessively bright indicia or "cold spots" of inadequately illuminated indicia. Because the display is illuminated by a number of lamps which provide uneven illumination due to their discrete nature and placement in recesses in the back surface of the panel, and because any cutouts present also affect the intensity of light transmitted from the lamps throughout the panel, it is necessary to somehow even the intensity of illumination over the indicia. In addition, it is desirable that the intensity of illumination of the indicia be uniform from one display to another. This evening of the illumination intensity, or "balancing" of the display panel substrate, is typically achieved by varying the thickness (and hence the transmissivity) of the diffuse translucent layer on the front of the panel substrate; with the layer typically being thicker immediately above each lamp and thinner remote from the lamps. Once the substrate is balanced, the front surface is then overcoated with an opaque layer, and the underlying translucent layer exposed in a pattern defining the indicia desired in the display, thereby completing manufacture of the display panel.
The accepted manufacturing practice for balancing display panel substrates of the type described above is both labor-intensive and craft-sensitive. Once the substrate has been coated on the back and side surfaces and recesses drilled for the lamps, the display is partially assembled so that it can be illuminated by the lamps (which are typically mounted on a circuit board behind the display panel). Working under low ambient light conditions, a skilled operator energizes the lamps to illuminate the display, then applies a diffuse translucent layer to the front surface of the substrate by spray painting. Typically, the operator first sprays paint on the front surface immediately above the lamp recesses; a process referred to as "spotting". After the spotting of the lamps is completed, the operator then sprays the remaining front surface of the substrate to achieve an even illumination of the desired intensity. Because of the low ambient light, the operator cannot see the paint as it is being applied, but must judge the effect of his actions by the appearance of the panel as coated with still-wet paint. Once the operator feels that the front surface of the substrate has been coated with the translucent layer to achieve the desired brightness and evenness of illumination, the painted panel is cured in an oven, and the brightness of the display is then measured using a photometer. If the display is too bright at any point, the painting process is repeated; if the display is not bright enough, the translucent layer must be decreased in thickness. Because the paint used for the translucent layer is formulated to possess a very high resistance to surface abrasion, sanding the translucent layer to thin it and achieve a greater brightness is very labor-intensive. The sanding operation, like the painting operation, must be performed in a darkened room so that changes in the brightness level can be seen, and the operation depends on the skill of the operator for its success. If the operator removes too much paint at any place on the front surface, that place must again be repainted and the paint once more cured before balancing can be reattempted. As can be seen from this description, balancing of illuminated display panel substrates by the method presently known to the art requires operators skilled in painting, sanding, and photometry; the working conditions of painting, dust from sanding, and low light make the task difficult to perform on a regular basis; and the cost per panel is high.
There is therefore a need for an apparatus and method for the balancing of illuminated display panels that will minimize the need for skilled labor and reduce the cost of balancing.